Known devices of this kind, for example, as shown in German Disclosure Specification DT-OS No. 1,906,330 and Deutsches Gebrauchsmuster DT-GM No. 7,218,872 ascertain the temperature of the road surface, or the temperature near the road surface, and also the relative humidity of the air, in order to draw from these values conclusions as to the possibility of the formation of glazed ice. The disadvantage of these systems is that they do not provide an indication as to whether the road is actually slippery. What is indicated is only that the conditions for sleet formation are given. Other known devices of this kind as set forth in German Disclosure Specification DT-OS No. 1,936,255 and DT-OS No. 2,229,386 employ an element mounted on the vehicle and in contact with the road surface for response to changes in the adhesion to the road surface. There is used, for example, a slide shoe held by a holding arm in contact with the roadway surface, the pulling force produced by the friction of the slide shoe on the road surface being measured and used as a measure for the adhesion between the surfaces of the slide shoe and the road surface. It is also a known practice to employ a roller or drag wheel being used as a measure of adhesion between the road and the drag wheel. The disadvantages of these known devices is that a road-engaging part in the form of a slide shoe or a drag wheel is required.
Still other known devices for determining the presence of glazed ice (see German Disclosure Specification DT-OS No. 2,416,623) take advantage of the difference in the rotary speed of a driven and a non-driven wheel. Such devices depend on the fact that the slippage between a wheel and the roadway is, in the case of a driven wheel of an automobile, always at least somewhat greater than for a non-driven, freely rolling wheel, and the fact that this slippage will become greater with increasing road slipperiness in a considerably higher degree on a driven or braked wheel than on a freely rolling wheel. A disadvantage of this arrangement is that the static radius of the wheels may vary with the wear, the air pressure and the load and may, therefore, necessitate frequent calibration. Torques of different values, depending upon the drive gear also influence the difference in the rotary speed, independently of road glaziness. A further disadvantage of such a device is the fact that, independently of road glaziness, differences in the rotary speed occur also when the wheels negotiate a curve.
U.S. Pat. No. 2,849,701 describes a highway condition indicating system comprising road signs mounted along the highway to signal the presence of weather conditions detected by stationary sensors. Such a system is not adapted for mounting on vehicles themselves to caution the vehicle of dangerous ice conditions wherever it travels.